import java.util.ArrayList;
import java.util.List;

public class Solution {
    //电话号码的字母组合
    String[] hash = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    List<String> ret;
    StringBuffer path;
    public List<String> letterCombinations(String digits) {
        ret = new ArrayList<>();
        path = new StringBuffer();
        if (digits.length() == 0) {
            return ret;
        }
        dfs(digits, 0);
        return ret;
    }

    public void dfs(String digits, int pos) {
        if (pos == digits.length()) {
            ret.add(path.toString());
            return;
        }
        String cur = hash[digits.charAt(pos) - '0'];
        for (int i = 0; i < cur.length(); i++) {
            path.append(cur.charAt(i));
            dfs(digits, pos + 1);
            path.deleteCharAt(path.length() - 1);//回复现场
        }
    }
}
